Because of elliptic curve mathematics, and the limits we use when interpreting these numbers, we call one of these even and the other odd, and apply the necessary prefix accordingly.
But why do we call them even or odds based on their sign? Why don't we call them negatives or positives? Just like in maths. Also, if we calculate the equation we will get 2 results as you said. One positive (y) and one negative (-y). Which of those do we choose? Does the computer chooses it randomly?
For a slightly more technical answer, to calculate the y coordinate, you use the following equation:
y2 (mod p) = (x3 + 7) (mod p)
What is
p? I know that
α mod β gives us the remainder of the division, but I haven't seen that again and I don't understand it.